Fuel-rich solid propellant gas generators are well-known in the art. Fuel-rich gas is gas which contains more than the stochiometric ratio of fuel to oxygen molecules based on the theoretical molecular oxygen requirement. Fuel-rich propellant systems presently considered for air-augmented rockets include metal slurries, metal-laden solid grains and hybrids of these two systems. Boron is one of the prime metallic fuel candidates. Boron slurry systems conventionally employ a hydrocarbon carrier fluid and an oxidizer such as chlorine trifluoride. A conventional solid grain system is one containing a hydrocarbon binder and an oxidizer such as ammonium perchlorate.
A solid propellant highly loaded with boron is a desirable propellant system for air-augmented rockets. However, with conventional non-energetic binders, solid oxidizer must be added to the fuel/binder composition to achieve satisfactory burning of the grain in the primary motor. As a result, processability becomes a problem. Presently, although boron loading of 60% has been reported with no particular processing difficulty, an expulsion efficiency problem has been encountered in these formulations due to the low oxidizer/binder ratio. The achievement of proper oxidizer/binder ratio is hindered by the minimum amount of binder required to attain satisfactory processability.
The dinitrofluoromethyl epoxide monomers employed according to the present invention are known compounds of the formula ##STR2## wherein R is alkylene of 1 to 4 carbon atoms. These monomers are prepared by first reacting an unsaturated aliphatic alcohol of the formula HO--R--CH.dbd.CH.sub.2 with 1,1-dinitroethylene to produce the corresponding unsaturated ether CH.sub.2 .dbd.CH--R--O--CH.sub.2 --C(NO.sub.2).sub.2 H wherein R is as defined above. The ether is then fluorinated and the resulting fluoroether is converted by direct oxidation with peracids of the formula R.sub.1 CO.sub.3 H wherein R.sub.1 is alkyl, trifluoroalkyl or aryl such as phenyl or benzyl to the above identified dinitrofluoromethyl epoxide monomer.
The synthesis of the dinitrofluoromethyl epoxide monomers employed according to the present invention is illustrated by the preparation of glycidyl 2,2-dinitro-2-fluoroethoxide. The first step in the synthesis consists of the preparation of allyl 2,2-dinitroethyl ether according to the procedure set forth in J. Org. Chem., 23, 813 (1958). This preparation consists of the addition of allyl alcohol to 1,1-dinitroethylene. 1,1-Dinitroethylene is a reactive intermediate generated in situ from 2-bromo-2,2-dinitroethyl acetate, 1,2-dichloro-1, 1-dinitroethane or 1,1,1-trinitroethane. 1,1-Dinitroethylene can be generated from 1,2-dichloro-1,1-dinitroethane according to the procedure set forth in J. Org. Chem., 31, 369 (1966). The next step in the synthesis is the fluorination of allyl 2,2-dinitroethyl ether with perchloryl fluoride to form the corresponding allyl 2,2-dinitro-2-fluoroethyl ether. The fluorination is carried out in the presence of alkali. The final step of the synthesis consists in the epoxidation of the allyl 2,2-dinitro-2-fluoroethyl ether with peroxytrifluoroacetic acid to form the desired glycidyl 2,2-dinitro-2-fluoroethoxide. Other dinitrofluoromethyl epoxide monomers can be prepared by the above described synthesis by employing the appropriate alcohol such as 3-butene-1-ol, 2 -methyl-3-butene-1-ol or 2-propen-1-ol.
The above described dinitroethylation, fluorination and epoxidation reactions are conveniently carried out at about atmospheric temperature and pressure. The initial temperature of the dinitroethylation is usually about 15 to 25.degree. C. followed by agitation or stirring at room temperature for 10 to 15 hours. The starting temperature of the fluorination is reflux temperature followed by reaction at about 20.degree. to 25.degree. C. for about 3 to 4 hours. The epoxidation reaction is carried out at reflux temperature for about 1 to 3 hours. These reactions are usually performed in the presence of an inert solvent such as methylene chloride, carbon tetrachloride, chlorobenzene or chloroform.
The dinitrofluoromethyl epoxide polymers employed according to the present invention are known dihydroxy-terminated polyether polymers of the formula ##STR3## wherein R is as defined above and n is an integer from about 3 to 25. The dinitrofluoromethyl epoxide monomers are converted to liquid polymers of varying molecular weight by a catalytic reaction using a Lewis acid catalyst and an initiator containing hydroxy groups in the presence of an organic solvent. The polymerization is conducted by introducing the dinitrofluoromethyl epoxide monomer into a reaction flask containing solvent, adding a catalyst and then allowing the polymerization to proceed to the desired polymer. The course of the reaction can be followed by watching for the disappearance of the epoxy group. The polymerization can be terminated by adding water or any other suitable quenching material. The polymer is isolated from the reaction medium by first water extracting the solvent to remove the catalysts and then drying the extracted solvent and finally extracting the polymer with alcohol. The alcohol is evaporated to yield the polymer.
Suitable Lewis acid catalysts for polymerizing the dinitrofluoromethyl epoxide monomers are aluminum chloride, zinc chloride, ferric chloride, boron trifluoride hydrate and stannic chloride. Representative initiators containing an hydroxy group suitable for the epoxide polymerization are water, glycerine, glycols such as ethylene glycol, polypropylene glycol, polyethylene glycol, mixed polyethylene polypropylene glycols and glycerol. Suitable solvents include methylene chloride, carbon tetrachloride, ethylene chloride, methylene dichloride, methyl bromide and propyl chloride.